Limited Dissolve - All Boundaries
As from release notes, Limited Dissolve without All Boundaries will:


  Limited Dissolve will have no affect on edges or vertices selected on the openings, but will limit the mesh appropriately on the edges/vertices away from the openings. 


Limited Dissolve will affect only on edges / vertices which are located inside of closed areas (i.e. not closely to boundary loop).

All Boundaries checked will make tool to dissolve vertices located on the boundary loop if those vertices have no more than 2 edges connected to them.

Taking into consideration some examples, this will look as follows.

Unfolded cube with edge loops in the corners



Loops were removed by both options;  
All Boundaries removed several faces of the mesh. Those were faces which haven't any further extruding out; their boundary vertices had only 2 edges connecting them - the ones which the actual face consisted of.


UV sphere with several edges deleted



Edge loops in the top (and bottom) part of the mesh were decreased in both cases, surrounding faces were converted into Ngons;
All Boundaries option changed boundary loop (where mesh is opened); faces which had 1 vertex not connected to anything else were converted to tris, and that vertex dissolved.
If sphere weren't opened, there wouldn't be any differences between All Boundaries turned on or off.


Generally All Boundaries may seem a bit strange because it dissolves geometry fallen under the threshold on the flat surfaces and around opened loops; thus if there isn't any supported geometry remained, the result won't be as expected:


Unexpected result of dissolving with All Boundaries turned on; the plane subdivisions were removed and hence was removed that part of plane because only 2 edges were touching vertices in the corners of the mesh after dissolving.